A memory device using a semiconductor is a storage device which is implemented by using a semiconductor such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), or indium phosphide (InP). A semiconductor memory device is mainly classified into a volatile memory device and a nonvolatile memory device.
The volatile memory device is a memory device in which stored data dissipates when a power supply is cut. The volatile memory device includes a Static RAM (SRAM), a Dynamic RAM (DRAM), a Synchronous DRAM (SDRAM), and the like.
The nonvolatile memory device is a memory device in which stored data is maintained even when a power supply is cut. The nonvolatile memory device includes a Read Only Memory (ROM), a Programmable ROM (PROM), an Electrically Programmable ROM (EPROM), an Electrically Erasable and Programmable ROM (EEPROM), a flash memory device, or a resistive memory device (for example, a Phase-change RAM (PRAM), a Ferroelectric RAM (FRAM), or a Resistive RAM (RRAM)).
With an increase in a degree of integration of semiconductor devices, accordingly, design rules regarding elements of semiconductor devices, including memory devices, are being decreased. As a result, thickness of semiconductor layers, such as the thickness of a poly gate layer, can have an impact on the operation of the semiconductor memory device. As the thickness of the poly gate layer increases over the peripheral and cell area of a semiconductor memory device, the gate stack height increases as well. This increase in gate stack height can cause parasitic capacitance of the bit line.